1. Field of the Invention
The present invention relates to optical element holding apparatuses for holding optical elements, and more particularly, to an optical element holding apparatus for holding an optical element included in a projection optical system of an exposure apparatus.
2. Description of the Related Art
Reduction projection exposure apparatuses are generally used to transfer a circuit pattern formed on a mask onto a wafer or the like using a projection optical system in a manufacturing process of a micro device using a photolithographic technique.
The resolution of exposure depends on a wavelength of exposure light and a numerical aperture (NA) of the projection optical system. The resolution is increased as the wavelength is reduced and the numerical aperture is increased.
Therefore, recently, to comply with the demands for miniaturization of semiconductor devices, the wavelength of exposure light has been reduced. More specifically, the wavelength of the exposure light has been reduced in the order of extra-high pressure mercury lamp (i-line (wavelength is about 365 nm)), KrF excimer laser (wavelength is about 248 nm), and ArF excimer laser (wavelength is about 193 nm).
However, there is a limit to the miniaturization of semiconductor devices that can be achieved by lithography techniques using the above-described ultraviolet exposure light. Accordingly, to transfer an extremely fine circuit pattern of 0.1 μm or less with high resolution, a reduction projection exposure apparatus (hereinafter referred to as “EUV exposure apparatus”) using extreme ultraviolet (EUV) light having a wavelength of abut 10 nm to 15 nm, which is smaller than the wavelength of ultraviolet light, has been developed.
In the EUV exposure apparatus that performs exposure with high resolution, the projection optical system is required to have an extremely high accuracy. In addition, holding apparatuses are required to have sufficiently high natural frequencies.
FIG. 13 illustrates a holding apparatus described in Japanese Patent Laid-Open No. 2002-350699 (corresponding to U.S. Pat. No. 6,870,632). The holding apparatus holds an optical element 1 and includes three supporting devices 2 placed at three positions along the periphery of the optical element 1. Each of the supporting devices 2 includes bending members 7 and 9 disposed along the tangential direction with respect to the optical element 1 and a bending member 11 extending in a radial direction. The bending members 7, 9, and 11 are connected to the optical element 1 by an attachment portion 4, and are connected to a base structure 3 by a connecting portion 6. The bending members 7 and 9 are connected to each other by a rigid adapter 8. Thus, a sufficient natural frequency is provided.
The optical element must be held without redundant constraint in six degrees of freedom so that the optical element can be prevented from being deformed when external heat or force is applied. For example, in the case of holding the optical element at three positions, two degrees of freedom must be constrained at each of the three positions while leaving the remaining four degrees of freedom unconstrained.
However, in the above-described holding apparatus in which the bending members are arranged perpendicular to one another, the rigidity in the twisting direction at each holding section is not sufficiently small. Therefore, the optical element is redundantly constrained. In such a state, deformation caused by disturbance is transmitted to the optical element, which leads to reduction in the optical performance.